Python Decimal money: parse decimal text and choose rounding explicitly

Operation contract

The invoice fixture constructs Decimal from text and quantizes to two places with ROUND_HALF_UP. It separately rejects non-finite values. Decimal("12.345") rounds to 12.35 under that stated policy; converting a binary float first could carry its earlier approximation into the decimal object.

Failure and ownership boundary

Rounding is a domain decision. A tax calculation may require another policy or round at another stage. Decimal does not supply currency identity, input length limits, positive-value rules or unlimited arithmetic precision. The local context pins working precision for the bounded example. Python input exercise: accept an explicit integer grammar and Java BigDecimal: decimal amounts and explicit rounding address comparable choices.

Working program

from decimal import Decimal, localcontext, ROUND_HALF_UP

def invoice_amount(text):
    if len(text) > 24:
        raise ValueError("amount too long")
    with localcontext() as context:
        context.prec = 28
        amount = Decimal(text)
        if not amount.is_finite():
            raise ValueError("finite amount required")
        return amount.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)

print(invoice_amount("12.345"))
try:
    invoice_amount("NaN")
except ValueError:
    print("non-finite amount rejected")

Output

12.35
non-finite amount rejected

Costs and limits

Decimal work depends on retained digits and precision. This fixture caps textual length and context precision; it does not define every currency’s minor-unit or maximum amount rule.

Common Mistakes

• Construct exact decimal input from text.
• Do not leave rounding policy implicit.

Connected lessons

Python functions: define units, validate input and return a result, Python input exercise: accept an explicit integer grammar, Java BigDecimal: decimal amounts and explicit rounding.

Apply this boundary

NumPy broadcasting: declare axes and bound integer arithmetic, Pandas nullable integers: missing amounts are not zero.

Follow the ownership and update boundary

Python Fraction: exact ratios require an exact input representation.

Follow the integrity boundary

Python Decimal Inexact trap: reject fractional minor units.

Continue with Python amount parsing: define a wire format before Decimal.

Continue with Python decimal localcontext: keep a rounding rule inside one calculation.